Feature/customizable be nav

Hey guys

As promised at the Contao Camp 2012 I am working on a customization feature for the back end navigation.

I'm not opening this pull request because my version is ready to be merged. On the contrary, it's very far from it :-) I am doing this for several reasons:

* As it is public now, it puts some pressure on me so I'll get it finished ;-)
* I'll need some help by designers. We need a cool interface and probably a self-explanatory icon to switch between views
* I want to develop in public so everybody can follow and comment
* There's no ticket for that feature yet - now there is :-)

So there are a lot more commits to follow. If you want to contribute, hit me up on Skype or the IRC channel. Pull requests on my branch are very welcome although it's currently still a too early stage for one ;-)